This invention relates to x-ray system calibration, and more particularly relates to calibration of positional relationships among key components of an x-ray imaging machine.
The controlled delivery of x-rays on a diagnostic imaging system requires specific knowledge of the geometry, orientations, and positional relationships among the key components of the image chain. In order to collimate the x-ray beam, it is necessary to know the source to image distance (SID) as well as the actual size of the receptor that the beam is to be projected onto. Other parameters, such as the object-to-detector distance (ODD), are often required for computations involving automatic tracking of the tube and detector. The incorporation of these parameters in the control algorithms for the system may be accomplished in a variety of ways, including direct entry for situations where the geometry is fixed or by some form of calibration in cases where the geometry is variable or where component production tolerances could contribute to system performance variation. On film-based image systems, this calibration may involve taking direct measurements of the positioner and entering these into the system database or indirect means using x-ray images where certain dimensional data from the images are then entered into the system database. Both of these approaches involve a manual process that may result in non-repeatable outcomes. This problem is addressed by this invention, and a solution is provided.
The preferred embodiment is useful in an x-ray imaging machine including an x-ray tube generating x-rays from a focal spot. The x-ray field contains a central axis or ray, and the machine includes a detector of the x-rays defining a source image distance measured parallel to the central axis between said focal spot and said detector. The machine also includes collimator blades defining a blade plane and defining a focal distance measured parallel to the central axis between the focal spot and the blade plane. In such an environment, the machine may be calibrated by providing a phantom positioned between the detector and the x-ray tube. The phantom comprises a first portion defining a first dimension that generates on the detector a first x-ray image that defines a first projected dimension. The phantom also comprises a second portion defining a second dimension that generates on the detector a second x-ray image that defines a second projected dimension. A predetermined distance measured parallel to the central axis separates the first and second portions. A processor is responsive to the first projected dimension or the second projected dimension to calculate at least one of the focal distance and the source image distance.
Another embodiment also useful in the above-described environment comprises a method including positioning a phantom between the detector and the x-ray tube. The phantom comprises a first portion defining a first dimension and comprises a second portion defining a second dimension. A predetermined distance measured parallel to the central axis separates the first and second portions. A first x-ray image is generated on the detector in response to said first portion. The first x-ray image defines a first projected dimension. A second x-ray image is generated on the detector in response to said second portion. The second x-ray image defines a second projected dimension. At least one of said focal distance and said source image distance is calculated in response to the first projected dimension or the second projected dimension.
By using the foregoing techniques, an x-ray machine may be calibrated with a degree of convenience and accuracy previously unavailable. For example, eliminating the direct measurement of physical sizes and entering these values into the system database improve the consistency of the final system calibration. In this way, system performance and system-system performance consistency both are improved. In addition, long-term system quality is improved since a calibrated ODD value would be system specific and immune to error if the physical system configuration changes without a corresponding software database change.